[libvirt] [PATCH 1/2] logical: Use correct syntax for thin/sparse pool creation

[John Ferlan]

On 12/16/2014 10:20 AM, Daniel P. Berrange wrote:
> On Fri, Dec 12, 2014 at 04:14:29PM -0500, John Ferlan wrote:

<...snip...>

> 
> I'm unclear still on what the difference is between a thin snapshot (with
> no backing volume) and a thin volume ?

I've asked that question of someone from lvm...

The thin shapshot has a 'hidden' backing pool of sorts (seen with lvs -a)

# lvs LVM_Test -a
...
  test                   LVM_Test swi-a-s---   4.00m                [test_vorigin]   0.00                          
  [test_vorigin]         LVM_Test owi-a-s---   8.00m                                                               
...

When the thin snapshot is removed, the [test_vorigin] is removed as well.
The target file created (/dev/LVM_Test/test) will have the 8M size as seen
via the virsh vol-list --details or virsh vol-info commands. Compare that
to the thin lv's which have the allocation size listed for capacity until patch 2
is added which asks the pool for it's size (e.g. the --virtualsize value).

> 
> FWIW, the original intent was that this provide a volume that is equivalent
> semantically to a sparse file created on a filesystem. ie the LVM equivalent
> to  'dd if=/dev/zero of=foo.img seek=1G count=0'
> 
> 

Given Jan's point/concern - I'll post a patch that adds the "--type snapshot"
until more time/effort can be devoted to thin pool support.

John